U.S. patent number 4,820,916 [Application Number 07/046,075] was granted by the patent office on 1989-04-11 for optically powered sensor system.
This patent grant is currently assigned to Simmonds Precision Products. Invention is credited to Douglas R. Patriquin.
United States Patent |
4,820,916 |
Patriquin |
April 11, 1989 |
Optically powered sensor system
Abstract
An optically powered sensor system includes a plurality of
sensors connected to a system optical bus that communicates with a
system controller. Optical energy is transmitted along the bus for
distribution to all sensors in the system with return pulses from
the various sensors transmitted on the bus to the system
controller. Each sensor includes a photodiode array for converting
optical energy transmitted system-wide by the controller into
electrical energy for storage in a storage capacitor associated
with each sensor. A transducer, such as a thermistor, and a
fixed-value reference, such as a resistor, are connected to a pulse
encoder and, in response to power switched from the capacitor, the
pulse encoder produces a series of short-duration pulses having a
pulse spacing that is dependent upon the fixed value of the
reference and the parameter-affected value of the transducer. The
pulses are used to drive an optical source for transmitting optical
pulses from the sensor to the system controller. The parameter
value is determined by multiplying the known value of the reference
resistor by a time factor ratio which is related by the reference
pulse timing and the parameter-affected pulse timing. An optical
sensor system is provided in which the measurement of a parameter
by each sensor is immune to variations in the electrical components
at each sensor.
Inventors: |
Patriquin; Douglas R.
(Middlebury, VT) |
Assignee: |
Simmonds Precision Products
(Tarrytown, NY)
|
Family
ID: |
21941464 |
Appl.
No.: |
07/046,075 |
Filed: |
May 5, 1987 |
Current U.S.
Class: |
250/208.2;
250/227.11; 250/578.1 |
Current CPC
Class: |
H04B
10/807 (20130101) |
Current International
Class: |
H04B
10/00 (20060101); H01J 005/16 (); G01D
005/34 () |
Field of
Search: |
;250/231R,231P,227,551
;350/96.15,96.16,96.17
;455/602,605,606,607,608,610,612,617,613 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Nelms; David C.
Assistant Examiner: Messinger; Michael
Attorney, Agent or Firm: Cushman, Darby & Cushman
Claims
What is claimed is:
1. An optically powered sensor system comprising:
an optical energy source for transmitting optical energy;
an optical pathway for conveying the optical energy transmitted by
said optical energy source;
at least one sensor connected to said optical pathway for receiving
optical energy from said optical energy source, said sensor
including means for converting optical energy from said optical
pathway to electricity and storing the electrical energy, a
transducer responsive to a selected parameter and having an
electrical characteristic that varies as a function of the selected
parameter, a reference component having an electrical
characteristic that is independent of the selected parameter,
circuit means for providing electrical energy from said converting
and storing means to said transducer and said reference component
and producing a multi-pulse output having a first time attribute
that is a function of the electrical characteristic of said
transducer and a second time attribute that is a function of the
electrical characteristic of said reference component.
2. The optically powered sensor system of claim 1, further
comprising:
means for converting the pulse output of said circuit means to an
optical pulse output.
3. The optically powered sensor system of claim 1, further
comprising:
means for converting the pulse output of said circuit means to an
optical pulse output and for introducing the optical pulse output
onto said optical pathway.
4. The optically powered sensor system of claim 1, wherein said
transducer comprises a temperature-responsive resistor and said
reference component comprises a fixed-value resistor.
5. The optically powered sensor system of claim 1, wherein said
circuit means produces at least a first set of two pulses, the
interpulse spacing of which is a function of the electrical
characteristic of said transducer, and another set of pulses, the
interpulse spacing of which is a function of the electrical
characteristic of said reference component.
6. The optically powered sensor system of claim 5, wherein the
pulses of said first and second sets of pulses are fixed-duration
pulses.
7. The optically powered sensor system of claim 1, wherein said
circuit means further comprises a first pulse generator for
generating pulses having pulse widths that are a function of the
transducer electrical characteristic and of the reference component
electrical characteristic and a second pulse generator connected to
said first pulse generator for providing fixed-duration pulses in
response to the leading and trailing edge transitions of said
pulse-width modulated pulses.
8. The optically powered sensor system of claim 7, wherein said
circuit means further comprises:
means for selectively switching electricity from said converting
and storing means between said transducer and said reference
component.
9. The optically powered sensor system of claim 8, wherein said
circuit means further comprises:
a bi-state latch between said first and second pulse generators,
said latch changing state with the leading and trailing edge
transitions of the pulse-width modulated pulses to trigger said
second pulse generator.
10. An optically powered sensor for sensing a parameter and
providing a pulse output representative of the sensed parameter,
comprising:
means for converting optical energy to electricity and storing the
electrical energy, a transducer responsive to a selected parameter
and having an electrical characteristic that varies as a function
of the selected parameter, a reference component having an
electrical characteristic that is independent of the selected
parameter, circuit means for providing electrical energy from said
converting and storing means to said transducer and said reference
component and producing a multi-pulse output having a first time
attribute that is a function of the electrical characteristic of
said transducer and a second time attribute that is a function of
the electrical characteristic of said reference component.
11. The optically powered sensor of claim 10, wherein said circuit
means produces at least a first set of two pulses, the interpulse
spacing of which is a function of the electrical characteristic of
said transducer, and another set of pulses, the interpulse spacing
of which is a function of the electrical characteristic of the
reference component.
12. The optically powered sensor of claim 11, wherein the pulses of
said first and second sets of pulses are fixed-duration pulses.
13. The optically powered sensor of claim 10, wherein said circuit
means further comprises a first pulse generator for generating
successive pulses having respective pulse widths that are a
function of the parameter-affected electrical characteristic of the
transducer and of the electrical characteristic of the reference
component and a second pulse generator connected to said first
pulse generator for providing fixed-duration pulses in response to
the leading and trailing edge transitions of said pulse-width
modulated pulses.
14. The optically powered sensor of claim 13, wherein said circuit
means further comprises:
means for selectively switching electricity from said converting
and storing means between said transducer and said reference
component.
15. The optically powered sensor of claim 14, wherein said circuit
means further comprises:
a bi-state latch between said first and second pulse generators,
said latch changing state with the leading and trailing edge
transitions of the pulse-width modulated pulses to trigger said
second pulse generator.
16. The optically powered sensor of claim 10, further
comprising:
means for converting the pulse output of said circuit means to an
optical pulse output.
17. The optically powered sensor of claim 10, wherein said
transducer comprises a temperature responsive resistor and said
reference component comprises a fixed-value resistor.
Description
BACKGROUND OF THE INVENTION
The present invention relates to devices and systems for sensing
physical parameters, and, more particularly, to optically powered
sensors and sensing systems in which optical energy from a source
is provided to one or more sensors which, in turn, provide
information-bearing optical energy representative of the sensed
parameter.
Various types of sensors and sensor systems are known for measuring
physical parameters. Traditionally, electrical sensors which
provide a variation in resistance, capacitance, or other electrical
characteristics as a function of a sensed physical parameter have
been used to provide an electrical current or voltage output. For
example, the resistance of a thermistor varies as a function of its
temperature and can be used in a simple bridge circuit to provide a
temperature responsive output current. In a similar manner,
capacitors and capacitor-like structures can be used to provide
electrical signal outputs that are responsive to environmental
parameters that affect the dielectric constant of the capacitor. In
a system or network context, groups of sensors are typically
interconnected with a controller which provides source electrical
power to the various sensors and measures or otherwise senses the
parameter-affected electrical characteristic. In general,
electrical sensors and electrical interconnections represent highly
developed and reliable technology, although unshielded systems can
be subject to electromagnetic interference (EMI).
With the advent of optical fibers, sensor systems using optical
fibers to transmit information from one node in a network to
another have been developed or proposed. Optical fiber transmission
is best suited to digitally encoded optical pulses in which the
information to be conveyed is encoded by varying an attribute of
the pulse, such as the pulse width, amplitude, or repetition rate.
Efforts directed to the transmission of analog light through
optical fibers is less than optimal because of the substantial
variation in attenuation for the transmitted energy as a
consequence of the fiber temperature, external pressure applied to
the fiber, the presence of small-radius bends in the fiber, and the
cumulative effects of defects in the fiber.
In view of the highly developed state of traditional electrical
sensors and the advantages attendant to pulse transmission in
optical fibers, an optimal system can be achieved using traditional
electrical sensors with optical fiber interconnection. In general,
however, the need to power the electrical sensors requires separate
electrical power paths to the sensors and thus adds undesired
complexity to the overall system.
In one optical sensor system, as disclosed in U.S. Pat. No.
4,346,478 to Sichling, optical energy is transmitted via optical
fibers to a sensor which includes a photodetector and a storage
capacitor for converting the input optical energy to electricity
for storage in the capacitor. A transducer, such as a temperature
sensor, uses the stored electrical energy to provide an electrical
output to a pulse width modulator, such as a light emitting diode
or a laser diode, to transmit one or more return pulses indicative
of the measured parameter. While the Sichling system operates to
provide duration-modulated pulses, the overall accuracy of the
measurement is a function of the stored energy and the accuracy can
degrade with changing characteristics of the storage capacitor, as
can occur, for example, with changes in temperature and component
aging. In additional, the use of pulse width modulation requires
that the light emitting diode or laser diode be powered during the
transmission of the entire pulse. In the context of low-power
systems, the optical energy emitter can consume the major portion
of the available stored energy and represent a constraint to
efficient operation.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the present invention,
among others, to provide an optical sensor and sensing system in
which optical energy is stored as electrical energy for use in
sensing a measurable parameter.
It is another object of the present invention to provide an optical
sensor and sensing system in which optical energy is stored as
electrical energy for use in sensing a measurable parameter and in
which an optical signal is generated as a function of the measured
parameter.
It is still another object of the present invention to provide an
optical sensor and sensing system in which optical energy is stored
as electrical energy for use in sensing a measurable parameter and
in which an optical signal is generated as a function of the
measured parameter and as a function of a reference value.
It is a still further object of the present invention to provide an
optical sensor and sensing system for sensing a measurable
parameter and providing an optical pulse signal as a function of
the measured parameter and a reference value in which the reference
value serves as a basis for determining the measured value.
It is a still further object of the present invention to provide an
optical sensor and sensing system for sensing a measurable
parameter and providing an optical pulse signal as a function of
the measured parameter and a reference value in which optical pulse
generation occurs in an energy efficient manner.
In view of these objects, and others, the present invention
provides an optically powered sensor and system for measuring
various parameters and providing an optical signal representative
of the measured parameter. The optically powered system includes at
least one sensor having a power converter for converting optical
energy to electrical energy for storage in an electrical storage
device, such as a capacitor, associated with the sensor. A
transducer, such as a thermistor, and a reference unit, such as a
fixed-value resistor, is associated with each sensor. Electrical
current provided by the storage device is provided through the
transducer and the reference and controls a pulse encoder to
provide output electrical pulses that are a function of the
fixed-value reference and the parameter-affected measure value. The
pulse output, in turn, drives an optical energy source to provide
optical pulses having an attribute representative of the
fixed-value reference and the measured-parameter affected value of
the transducer.
In a preferred embodiment, an optically powered sensor system
includes a plurality of sensors connected to a system optical bus
that communicates with a micro-processor controlled system
controller. The system controller transmits optical energy along
the bus for system-wide distribution to all sensors in the system
and receives return pulses from the various sensors, the return
pulses including information as to the measured parameter value
sensed by the sensor and to a sensor specific reference value. Each
sensor incudes a photodiode array for converting optical energy
transmitted system-wide by the controller into electrical energy
for storage in a capacitor associated with each sensor. A
transducer, such as a thermistor, and a fixed-value reference, such
as a resistor, are connected to a pulse encoder. In response to
power switched from the capacitor, the pulse encoder produces a
series of short duration pulses having a pulse spacing that is
dependent upon the fixed value of the reference and the
parameter-affected value of the transducer. The pulses are used to
drive an optical source for transmitting corresponding short
duration optical pulses from the sensor to the system controller.
The parameter value is determined by multiplying the known value of
the reference resistor by a time factor ratio that is related to
the reference pulse spacing and the parameter-affected pulse
spacing.
In a multi-sensor system, each sensor is provided with
sensor-specific time delay prior to the transmission of return
pulses from the sensor to the system controller to allow a
predetermined 'time window ' for each of the sensors to effect
transmission to the central controller. The various sensors of the
system thus transmit return pulses in a predetermined
time-multiplexed sequence.
The present invention advantageously provides an optically powered
sensor system in which the value of the measured parameter is
obtained as a function of the reference value, and, accordingly,
the measurement of the parameter is independent of the energy
storage device used in each sensor and is also relatively immune to
component value drift. The use of short duration return pulses
minimizes the power requirement of the optical energy generator at
the sensor and results in substantial efficiency gains for the
system.
Other objects and further scope of applicability of the present
invention will become apparent from the detailed description to
follow, taken in conjunction with the accompanying drawings, in
which like parts are designated by like reference characters.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is an overall schematic diagram of an optical sensor system
in accordance with the present invention;
FIG. 2 is a schematic block diagram of an exemplary sensor in
accordance with the present invention;
FIG. 3 is a functional block diagram of a pulse code modulator for
providing pulses having attributes representative of a measured
value and a reference value;
FIGS. 4A to 4D represent a pulse chart illustrating the sequence of
operations for the sensor system of FIG. 1;
FIG. 5 is a flow diagram of the control sequence for the sensor
system to obtain the measured-value and reference-value pulses of
FIGS. 4A to 4D; and
FIG. 6 illustrates the linear correspondence for an output value
relative to a measured value input.
DESCRIPTION OF THE PREFERRED EMBODIMENT
An optically powered sensor system is shown in schematic form in
FIG. 1 and is designated generally therein by the reference
character 10. As shown, a plurality of sensors S.sub.O, S.sub.1, .
. . S.sub.n-1, S.sub.n are connected by respective optical fibers
f.sub.O, f.sub.1, . . . f.sub.n -1, f.sub.n to a distribution
coupler 12, which, in turn, is connected to another coupler 14
through an optical fiber bus 16. The coupler 14 is designed to
distribute optical energy from the bus 16 to the respective sensors
S.sub.n and, conversely, direct optical energy from the sensors
S.sub.n along the bus 16 to the coupler 14. An optical source 18,
such as a laser diode, is connected to the coupler 14 and directs
optical energy into the coupler 14 for transmission via the bus 16
to the coupler 12 and system-wide distribution to the various
sensors S.sub.n. In an analogous manner, an optical energy receiver
20, such as PIN diode, is connected to the coupler 14 and converts
optical energy provided from the sensors S.sub.n through the bus 16
into corresponding electrical signals. The couplers 12 and 14 can
take the form of star-type couplers or lateral couplings.
A system controller 22 is connected to the optical source 18 and
the optical receiver 20 and functions, as described below, to drive
the optical source 18 to introduce optical energy into the system
and to process the return signals from the optical receiver 20. The
controller 22 includes a logic unit 24 that operates through a
recurring cycle under the control of a clock 26 to provide current
through a drive amplifier 28 to cause the optical source 18 to
direct optical energy through the coupler 14 and the bus 16 to the
coupler 12 and the various sensors S.sub.n. Additionally, an
amplifier 30 accepts pulse signals from the optical receiver 20 for
presentation to the logic unit 24. The controller 22 operates under
the general control of a micro-processor 32 with communication
provided through a bus 34 and respective I/O ports (unnumbered). An
output device 36, such as a multi-digit display, is connected to
the micro-processor 32 and provides output information as to the
parameters sensed by the sensors S.sub.n.
The organization of a sensor S.sub.n is shown in functional block
form in FIG. 2. As shown, each sensor S.sub.n includes a transducer
40 that has a characteristic, such as resistance, which changes in
a predictable manner with the sensed parameter. A pulse encoder 42
is connected to the transducer 40 and to a fixed-value reference 44
and provides a pulse output, as described more fully below, to an
output driver 46 which, in turn, drives a laser diode 48 to provide
optical pulses through the coupler 12 and the bus 16. The
transducer 40 can take the from of a thermistor that changes
resistance with temperature, and the reference 44 can take the form
a precision, fixed-value resistor.
Optical energy provided by the optical energy source 18 through the
bus 16 and the coupler 12 is provided to a power converter 50
within each sensor S.sub.n which includes a series-connected array
of photodiodes 52 in shunt circuit with a storage capacitor 54 and
in series circuit with a diode 56. Optical energy provided through
the bus 16 and the coupler 12 is converted to a DC potential by the
photodiodes 52 and stored in the shunt-connected capacitor 54. The
diode 56 has a low forward voltage drop and serves to isolate the
photodiodes 52 and the capacitor 54 from the remainder of the
circuitry. A power switch 58 is connected to the power converter 50
and selectively provides power to the pulse encoder 42 under the
control of a timing circuit 60. The power switch 58 can take the
form of a MOSFET or a gate-triggered thyristor.
The pulse encoder 42 functions to provide a series of pulse
signals, as explained below in relationship to FIG. 4, that provide
information related to the measured parameter and the reference
value provided by the reference 44. As shown in FIG. 3, the pulse
encoder 42 includes a one-shot pulse generator 62 that controls a
flip-flop 66 which switches between set and reset states with
successive pulses provided by the pulse generator 62. The one-shot
pulse generator 62 is of the type that provides an output pulse of
varying duration in response to an input value, such as an input
resistance. A switch 64 is connected to the input of the one-shot
pulse generator 62 and is controlled by the timing circuit 60 to
connect either the reference 44 to the input of the one-shot pulse
generator 62 or the series-connected value of the reference 44 and
the transducer 40. The switch 64 can take the form, for example, of
an FET that is selectively gated to shunt the transducer 40. Thus,
the duration of the pulse output of the one-shot pulse generator 62
is controlled by the switch 64 to be of a first duration controlled
by the reference 44 only or a second value controlled by the sum of
the values provided by the reference 44 and the transducer 40,
which latter value is a function of the measured parameter. The
successive pulses of varying duration from the one-shot pulse
generator 62 cause the flip-flop 66 to alternate between its binary
HI or LO states with each transition. Another one-shot pulse
generator 68 is connected to the flip-flop 66 and is triggered on
each transition between the set and reset states to produce a
fixed, short-duration pulse with each state change.
The optically powered sensor system operates in accordance with the
flow diagram of FIG. 5 to sense the measured parameter and provide
a pulse sequence, as shown in FIG. 4C, containing information
representative of the measured parameter as well as the reference
value. As shown in FIG. 5, the system is initialized by
transmitting optical energy from the optical energy source 18
through the bus 16 for distribution to all sensors S.sub.n in the
system. The photodiodes 52 of each sensor S.sub.n convert the
distributed optical energy to an electrical current and charge the
capacitor 54 of each sensor S.sub.n to a full charge or near full
charge condition. In general, the optical energy provided from the
optical source 18 is of a sufficient duration and intensity to
assure a sufficient charge in the capacitor 54 of each sensor
S.sub.n. As shown by the graphs of FIGS. 4A and 4B, the distributed
optical energy is halted at time T.sub.O by the controller 22 in
response to commands provided from the micro-processor 32. The
termination of the distributed optical energy is detected by each
sensor S.sub.n and the power switches 58 of each of the sensors
S.sub.n are switched to apply power to the remaining sensor
circuitry. A current path is provided through the reference 44 of
each sensor S.sub.n with the resulting current fixed by the value
of the reference 44, and a current path is provided through the
respective transducer 40 with the current flow through the
transducer 40 responsive to the sensed parameter. For example,
where the transducer 40 is a thermistor, current flow provided
through the power switch 58 will be determined by the thermistor
temperature. Because of capacitive effects, power switching occurs
during a time period between T.sub.O and T.sub.l, as shown in FIG.
4B. The pulse encoder 42 of each sensor S.sub.n is provide with a
sensor specific time delay between time T.sub.1 and time T.sub.2 ;
in the preferred embodiment, the time delay is 450 microseconds.
Thus the time T.sub.1 to T.sub.2 is 450 microseconds for a first
sensor, 900 microseconds for a second sensor, 1350 microseconds for
a third sensor, etc. This sensor-specific time delay prior to the
transmission of return pulses from the sensor to the system
controller to allow a predetermined 'time window' for each of the
sensors S.sub.n to effect transmission to the system controller 22.
The various sensors S.sub.n of the system thus transmit return
pulses in a predetermined time-multiplexed sequence.
At the conclusion of the sensor-specific time delay between time
T.sub.1 and T.sub.2, the pulse encoder 42 is controlled by the
timing circuit 60 of the sensor S.sub.n to provide a series of
short-duration pulses containing information regarding the value of
the sensed parameter. As shown in FIG. 4C, the output of the pulse
encoder 42 is normally binary HI at time T.sub.2 and switches to
binary LO at time T.sub.3 and then returns to binary HI at time
T.sub.4. The transitions at time T.sub.3 and time T.sub.4 cause
successive state changes in the flip-flop 66 and trigger the
one-shot pulse generator 68 to provide two successive
short-duration pulses P.sub.1 and P.sub.2 (FIG. 4D) occurring at
and spaced by the time difference between time T.sub.3 and T.sub.4.
The pulse spacing between times T.sub.3 and T.sub.4 is determined
by the value of the reference 44 and is relatively fixed for all
the sensors S.sub.n. After switching to binary HI at time T.sub.4,
the pulse output remains at binary HI until time T.sub.5 when the
output switches to binary LO and then returns to binary HI at time
T.sub. 6. The transitions at time T.sub.5 and time T.sub.6 cause
successive state changes in the flip-flop 66 and trigger the
one-shot pulse generator 68 to provide two successive
short-duration pulses P.sub.3 and P.sub.4 (FIG. 4D) occurring at
and spaced by the time difference between times T.sub.5 and
T.sub.6. The time duration between times T.sub.4 and T.sub.5 is
determined by a combination of the value of the reference 44 and
the sensed value of the transducer 40. Accordingly and as explained
in more detail below, the pulse spacing between pulses P.sub.2 and
P.sub.3 is a function of the sensed parameter. The output remains
at binary LO from time T.sub.5 to time T.sub.6 at which latter time
the output returns to binary HI. The time duration between times
T.sub.5 and T.sub.6 is determined by the value of the reference
44.
The output of the pulse encoder 42 is provided to the output driver
46, which, in turn, drives the laser diode 48 to provide optical
return pulses P.sub.1, P.sub.2, P.sub.3, and P.sub.4 as shown in
FIG. 4D. The optical pulses P.sub.1, P.sub.2, P.sub.3, and P.sub.4
have a relatively short duration, that is, on the order of two to
eight microseconds, and thus constitute optical spikes rather than
pulses having an appreciable duration. The duration of the
electrical pulse output of the one-shot pulse generator 68 is
appropriate to drive the laser diode 48 to provide the
short-duration optical pulses P.sub.1, P.sub.2, P.sub.3, and
P.sub.4. The timing and spacing of the pulses P.sub.1 and P.sub.2
are determined by times T.sub.3 and T.sub.4, and the timing and
spacing of the pulses P.sub.3 and P.sub.4 are determined by the
time T.sub.5 and T.sub.6, and the interpulse duration between
pulses P.sub.2 and P.sub.3 is determined by the pulse P.sub.2 at
time T.sub.4 and the pulse P.sub.3 at time T.sub.5. The time
spacing between the pulses P.sub.1 and P.sub.2 is representative of
the fixed value of the reference 44 while the interpulse duration
between the pulses P.sub.2 and P.sub.3 is representative of the
combination of the fixed value of the reference 44 and the
parameter dependent value of the transducer 40.
As can be appreciated, the system operates to first transmit
optical energy from the controller 22 to all the optical sensors
S.sub.n in the system until the respective storage capacitors 54 of
each sensor S.sub.n are charged with sufficient energy to perform
the sensing function. After cessation of the charging optical
energy, each sensor S.sub.n waits a sensor-specific time interval
and then returns a series of short-duration pulses with pulses
P.sub.1 and P.sub.2 representative of a reference value, pulses
P.sub.2 and P.sub.3 representative of both the reference value and
the measured value, and pulses P.sub.3 and P.sub.4 representative
of the reference value. The inclusion of the reference pulse
information allows evaluation of the measured value in the context
of a reference that is subjected to the same power converter
variables, i.e., the storage capacitors 54, as the transducer 40 so
that errors introduced at the sensor S.sub.n by sub-optimal
performance of the energy storage function will affect both the
reference value determining pulses P.sub.1 and P.sub.2 and the
measured value pulses P.sub.2 and P.sub.3 and any errors will be
effectively cancelled.
The optical pulses returned by the laser diode 48 of each sensor
S.sub.n along the bus 16 to the controller 22 and the
micro-processor 32 by multiplying the known value of the reference
44 by a time factor ratio which is related by the reference pulse
time as presented in equation 1.
where
MV=desired measured value
RV=known reference value
T.sub.re f=reference value determined from pulse time difference
between pulse P.sub.1 at time T.sub.3 and pulse P.sub.2 at time
T.sub.4
T.sub.mv =measured value pulse time difference between pulse
P.sub.2 at time T.sub.4 and pulse P.sub.3 at time T.sub.5.
The value of the measured parameter is thus obtained as a function
of the reference value, and, accordingly, the measured value that
is representative of the measured parameter is independent of the
capacitance 54 used in each sensor S.sub.n and is also immune to
capacitive drift. In computing the spacing for T.sub.ref, the pulse
spacing of the first reference times T.sub.3 and T.sub.4, the pulse
spacing of the second reference times T.sub.5 and T.sub.6, or the
averaged value can be used. Once the measured value MV is
determined, the actual value of the measured parameter can be
obtained by multiplication by an appropriate scale factor.
The system of FIGS. 1 and 2 has demonstrated substantial linearity
in the case of resistance-based transducers. As shown in FIG. 6,
sensed input, in Kohms, is relatively linear with the indicated
output resistance, also in Kohms.
As can be appreciated, the present invention advantageously
provides an optically powered sensor system in which the value of
the measured parameter is obtained as a function of a reference
value, and, accordingly, the measurement of the sensed parameter is
independent of the energy storage device used in each sensor and is
also relatively immune to component value drift. The use of
short-duration optical pulses results in improved energy
utilization in contrast to systems which use pulse width
modulation.
Thus it will be appreciated from the above that as a result of the
present invention, a highly effective optically powered sensor
system is provided by which the principal objective, among others,
is completely fulfilled. It will be equally apparent and is
contemplated that modification and/or changes may be made in the
illustrated embodiment without departure from the invention.
Accordingly, it is expressly intended that the foregoing
description and accompanying drawings are illustrative of preferred
embodiments only, not limiting, and that the true spirit and scope
of the present invention will be determined by reference to the
appended claims and their legal equivalent.
* * * * *